This project aims to understand the role of intermolecular forces in biological phenomena. A major topic has been the measurement of forces between phospholipid bilayer membranes immersed in water. We are also measuring intermolecular forces between lipids in the same membrane. We have discovered a new class of "hydration" forces that appear to dominate the interaction of biological membranes at distances less than 30A. These forces are independent of electrical charge on the membrane surface and drop off exponentially with a decay constant about the dimension of a water molecule. It is these forces, reflecting work of removing water from the cell surface, that seem to control physical contact between membranes. We have measured the deformability of membranes and succeeded in forcing aggregates of molecules to go through phase transitions while measuring the work of creating the transition.